Magnetic induction well-logging instrument



5 Sheets-Sheet l INVENTOR. ROBERT A. BRODING BY v ATTORNEY ll llf I-llllJan 13, 1953 R. A. BRODING MAGNETIC INDUCTION WELL-LOGGING INSTRUMENTFiled Oct. 28, 1950 Jan. 13, 1953 R. A. BRODING MAGNETIC INDUCTIONWELL-LOGGING INSTRUMENT Filed 001:. 2a, 1950 5 Sheets-Sheet 2 |,oooCYCLES INVENTOR. ROBERT A. BRODING ATTORNEY Jan. 13, 1953 R. A. BRODINGMAGNETIC INDUCTION WELL-LOGGING INSTRUMENT Filed on. 28, 1950 3Sheets-Sheet 3 INVENTOR. ROBERT A. BRODlNG ATTORNEY Patented Jan. 13, 1953 MAGNETIC INDUCTION WELL-LOGGING INSTRUMENT Robert A. Broding,Dallas, Tex., assignor, by

mesne assignments, to Schlumberger Well Surveying Corporation, Houston,Tex., a corporation of Delaware Application October 28, 1950, Serial No.192,751

15 Claims. 1

This invention relates to geophysical exploration, and more particularlyto improvements in the simultaneous and separate determination ofelectrical and magnetic characteristics of the medium penetrated by abore hole. This invention is an improvement upon the system disclosed inmy Letters Patent 2,535,666, granted December 26, 1950, upon applicationSerial No. 72,451 for Electrical Logging System, filed J anuary 24,1949.

In geophysical exploration, the values of, or the variations in, any oneof several physical and electrical properties of the various mediaforming strata in the earth are susceptible of measurement. Many systemsare known for making measurements of one or more of these proper ties.However, much has been left to be desired in systems for measuringvariations in electrical resistivity and magnetic permeability ofsubsurface formations, particularly in the accuracy and resolution oftwo characteristics, as conductivity and susceptibility.

In carrying out the invention in one form thereof, there is providedinductive coupling means suitably suspended in a bore hole for movementadjacent earth formations. Alternating currents of dilfering frequencyare applied to the inductive coupling means for magnetic coupling withthe earth formations. By means of a detector responsive to onefrequency, the in-phase component of alternating current is measured toprovide an indication of the electrical conductivity of the earthformations, while a second detector responsive only to the otherfrequency is utilized to measure the out-of-phase component thereof toindicate the magnetic susceptibility of the earth formations. Bothmeasurements take place concurrently, thus eliminating any positionalerror which might otherwise occur in systems where successivemeasurements are made of the earth formations. There is at all timespositional correlation of the two parameters, electrical conductivityand magnetic susceptibility, with respect to the formations throughwhich the bore hole extends.

In accordance with this invention, these measurements of the changes intwo parameters are made with high resolution so as to be representativeof any abrupt local variations to minimize indefiniteness as to theexact position of the abrupt local variation. While having highprecision, the system is not disturbed by bore hole contaminations suchas small bits of metal.

More specifically in accordance with this invention, changes in magneticsusceptibility and electrical conductivity are measured and indicated asa function of the position of the measuring means along the bore hole.There is preferably included in an arm of at least one balanceablenetwork an induction coil inductively coupled to the medium surroundingit, as to the strata around the bore hole. Alternating currents of twodifferent frequencies may be applied to a single inductance coil, or twocoils may be provided, to each of which there is applied alternatingcurrent of different frequency, each coil being connected in its ownbalanceable network. By means of suitable filters the alternatingcurrents may be derived from a single source of multiple frequency, eachoutput circuit of each network being connected to a detector responsiveto only one of the selected frequencies, or separate sources ofdifferent frequency may be utilized.

' In one embodiment, the system for carrying out the present inventionincludes an exploration unit, sometimes referred to as a bomb, carryingthe balanceable circuit means and a plurality of phase-sensitive,frequency-selective detectors, with the inductive means arranged to bemagnetically coupled to the surrounding medium. The inductive means isconnected o the balanceable circuit means to cause an unbalance thereofrepresentative of changes in impedance. The balanceable circuit means isconnected to the alternating voltages of different frequencies, andprovides signals to the detectors representative of the change inimpedance of the inductive means. The detectors measure in-phase andout-of-phase components thereof as indi cations of changes in magneticsusceptibility and electrical conductivity.

In another embodiment of this invention, the separate response to eachof a plurality of alternating voltages is enhanced by a circuitconsisting of a plurality of Wheatstone bridges and an inductance coilwith a plurality of windings thereon, each balanceable bridge beingconnected to a separate winding of the inductance coil. In this mannerthe circuit for measurement of each parameter is substantially separatedfrom the influence of the other circuit, and each winding can be sodisposed and arranged on the inductance coil to provide maximumsensitivity for the parameter Whose changes are being measured by thatwinding.

For a more detailed explanation of the invention and for further objectsand advantages thereof, reference may now be had to the followingdescription taken in conjunction with the drawings in which:

Fig. 1 is a schematic diagram of one embodiment of the inductance coilof the present invention, illustrating its inductive coupling to themedium surrounding the bore hole;

Fig. 2 is a schematic diagram of the equivalent transformer illustratedin Fig. 1;

Fig. 3 is a diagrammatic view of an embodiment of the inventionutilizing a single-winding inductance coil and a single-bridgebalanceable circuit;

Fig. 4 is a schematic diagram of a two-winding inductance coilinductively coupled to the medium surrounding the bore hole; and

Fig. 5 is a diagrammatic view of'a further'embodiment of the inventionhaving a two-winding inductance coil and a two-bridge balanceablecircuit.

Referring now to Fig. 1, an inductance coil assembly i8 is shownpositioned in a borehole ll adjacent an earth formation [2 located justbelow an earth formation [3 having different electrical characteristics.Conductors l4 and I5 extend upwardly from winding lllb. The turns ofwinding liib rest against insulation Ina which separates the turns ofthe winding lllb from a core we of material of high permeabilityselected from materials suitable for magnetic cores at the frequenciessupplied to the coil ill. Powdered iron is one suitable material for thecore. A core of high-permeability material increases the magnetic fiuxflowing through the coil assembly I and material l2, and increasessensitivity to external changes, 1. e., to changes in the material ifsurrounding the coil 10, because the core 100 removes practically all ofthe reluctance inside coil 19 and leaves only the reluctance of theexternal path through material I2.

The winding IE1) is relatively long compared with the diameter of thebore hole in order that the current sheath linked thereto in thesurrounding medium or earth formation shall be as large as possible.This current sheath is schematically represented in Fig. 1 as asingle-turn secondary winding 16 including series resistance ll. Becauseof the length of the current sheath and the nature of the currentflowing in the surrounding earth formation, the equivalent secondaryresistance I! will markedly vary with variations in the electricalconductivity thereof. Likewise, because of the reluctance of themagnetic path through material I2, there'will be a change in thereactance of winding "JD with changes in magnetic susceptibility ofmaterial 12, causing in induced reactive component to be added to theimpedance of winding lllb.

7 By applying to conductors l4 and I alternating currents ofsubstantially differing frequency, one a multiple of the other or notharmonically related to each other, substantial improvement insensitivity and measurement of resistivity and susceptibility of theearth formations is accomplished. The reasons for the improved operationwill now be developed in terms of an analysis of the equation for theimpedance seen in. the primary winding of a transformer. In Fig. 1,winding iilb is the primary winding and an equivalent secondary isrepresented by single-turn winding it. The loading of winding lllb dueto its coupling to the adjacent earth formation is represented asresistor H in series with winding 16. This'transformer circuit can beshown schematically as in Fig. 2, where:

e equivalent generator voltage of the source supplying winding lllb;

The effect of the magnetic susceptibility of a surrounding earthformation is primarily upon the inductance L of primary winding [01).The equation for this relation has the following form:

L=1.25N P X 10- henries (1) where N=number of turns :permeance of themagnetic paths in coil l0 and in the surrounding medium, and is afunction of magnetic susceptibility and magnetic flux paths.

Since X11:7'wL11,'where i=mathematical operation symbol indicatingquadrature phase relation, w=21r times frequency, and Lnzprimaryinductance, X11 can be expressed approximately as:

Xn iw 1.25'N PX10- reactance ohms (2) While the absolutevalue of X11 maydiffer somewhat from this equation, the important fact is thatit varieslinearly with permeance P and thus varies linearly with magneticsusceptibility.

' The eifect of the electrical conductivity of the surrounding medium isprimarily upon th equivalent secondary coil 13 and resistor ii. Theequation for the equivalent transformer of Fig. 2 has the followinggeneral form:

Inthe special case of a coil in atcircuit with metal in its magneticfield, Z12 will be -y'wM, where M=mutual inductance. Then, substitutingfor Z12 and Z2; in Equation 3:

By rationalizing and simplifying the second term of Equation 4, there isobtained:

For purposes of analysis of performance, the resistive expression R11will be ignored because Rn-is predominantly the resistance of bridge 23of Fig. 3 and is not affected by movement of said unit if: in-bore hole3 i With R11 omitted,-

Changes in X11 have already been shown to depend primarily upon magneticpernieance and hence. upon the magnetic susceptibility of the materialinthe magnetic flux path.

In the case of winding 423b, R22 is resistor IT and Lczls the inductanceof winding 16. Hence,

changes in electrical conductivity will change R22 in the term w M R zi'20 Since the inductance L22 of coil i6 is very small, the factor (wL22)can be ignored at audio frequencies. This allows an approximateexpression for the induced resistive component as from which it can beseen that the induced resistive component will vary directly withfrequency squared and inversely with the value of resistor ll. Thus, theuse of a higher frequency for measurement of the effect on winding 10bthat are induced by changes in electrical conductivity will increasesensitivity to such changes by a factor equal to the square of theincrease in frequency.

Since inductance L22 of winding I3 is very small, the induced reactancedue to the third term of Equation 6, namely,

j 22 Rid-( 20 can be neglected.

Now that the basic theory underlying the invention has been set forth,the embodiment illustrated in Fig. 3 will be described in detail. Thewinding lilb is disposed in bore hole II and is relatively long comparedwith the diameter of bore hole H. In order to accentuate the responsedue to changes in resistance of the earth formations, winding lilb isenergized with alternating current of high frequency, of the order of10,000 cycles per second. Any suitable source 49 of such high frequencymay be utilized. It is supplied to winding [b by way of a bridge 20. Alow-frequency alternating current is also supplied to winding b in orderto produce the impedance variation for measurement of susceptibility.Any suitable low-frequency source 48 may be used to energize windingI02) by way of bridge 20. By utilizing the different frequencies,greater resolution of impedance changes is obtained with greater ease ofseparation of the two for measurement of the iii-phase component of the10,000 cycle voltage for resistance and the out-of-phase component ofthe 1,000 cycle volt age for susceptibility. As can be seen fromEquation 2, there is minimum loss of sensitivity by the use of the lowerfrequency current for measurement of susceptibility since frequencyappears only to the first power. Hence, in accordance with the presentinvention maximum overall sensitivity is obtained for the simultaneousmeasurements of resistance and susceptibility by the use of the twoalternating currents of different frequency, the higher frequency beingused for the resistance measurement, indicative of conductivity.

The energization of winding liib is by way of the balanceable network20, of the Wheatstone bridge type, which bridge is unbalanced inaccordance with variations in conductivity and susceptibility of theearth formations.

When the bridge 20 is unbalanced, a filter 22 applies to an amplifier 23the lower frequency 1,000 cycle voltage, excluding the higher frequencyvoltage. The output of amplifier 23 is applied to a phase-sensitivefrequency-selective detector 24 by way of a transformer 24d. While thoseskilled in the art may utilize any suitable 6 detector, there isillustrated one of the type in which a voltage of the lower frequency of1,000 cycles per second is applied by filter 25 to a phase-shifter 26and thence across a diagonal,

or to the center taps, of the detector 24. The phase-shifter 26 isadjusted until the voltage applied to detector 24 is in phase with thequadrature component of the voltage applied by way of the secondarywinding of transformer 24d.

The quadrature component of the 1,000 cycle voltage applied bytransformer 24d from bridge 20 varies with the susceptibility of theearth formations and the corresponding unbalance of bridge 20. Hence,the rectifiers 24a and 24b of of the detector produce across the output,or loading resistor 240, a unidirectional or directcurrent output whosemagnitude will be proportional to the induced reactive component whichchanged the impedance of winding Hlb to give rise to the bridgeunbalance. As already explained, that reactive component varies withsusceptibility and hence the detector output may be connected directlyto an indicating and/or recording instrument, or, as shown, by way of anamplifier 45 to a recorder 30. The record sheet or recorder 30 hasassociated therewith two markers, the first producing on the sheet arecord 30s of change of susceptibility and the other a record 300 of thechange in conductivity of the earth formations. The manner in which thelatter marker is driven is explained in the immediately followingdescription. The record sheet will be driven in correlation withposition of the winding 10b in the bore hole for logging in relation todepth the two desired parameters.

When the bridge 20 is unbalanced, there is applied to amplifier 32 byway of filter 3| the alternating current voltage of 10,000 cycles persecond, the filter 3| excluding therefrom the 1,000 cycle voltage. Theoutput of amplifier 32 is applied to detector 33, similar in design andoperation to detector 24, and including rectifiers 33a and 33b, outputresistor 33c and a source through filter 34 and phase-shifter 35 ofalternating current of 10,000 cycles per second. Since it is theresistive component to which detector 33 is to respond, phase-shifter 35is adjusted so that the voltage applied between the center taps will bein phase with the in-phase component of the voltage applied by thesecondary winding of transformer 33d to detector 33. Hence, there isproduced a direct-current output whose magnitude will vary with theinduced component of resistivity of winding lilb which in turn willdepend upon the conductivity of the earth formation with which it isinductively coupled. While the output of detector 33 may be directlyconnected to indicating and/or recording apparatus, it is shown applyingits output voltage to an amplifier 46 for operation of the marker toproduce a record or trace 30c on the record-chart of changes inconductivity of the earth formations in correlation with the depth ofthe bore hole II.

In the foregoing description of the operation of Fig. 3 it was tacitlyassumed the balanceable circuit 20 had been adjusted for properoperation. The balancing of bridge 20 is relatively easy to accomplishby proper adjustment of the values of variable resistor 20b and ofvariable capacitor 200. With inductance coil [0 suspended in air, bridge20 is balanced for minimum, near zero, output for both thehigh-frequency and the low-frequency voltages. This requires adjustmentof both the resistor 20b and capacitor 200 for balance of the resistiveand reactive components of the bridge.

After bridge 20 is balanced, the phaseeshifters 26 and 35 are thenadjusted, one for quadrature and the other for in-phase measurement. Atwo or three-turn loop of copper wire, with a resistor in seriestherewith having a resistance which is high compared to the reactance ofthe. loop, such as two to three thousand ohms,-is then slid aroundwinding [b to be closely coupled thereto. Such a load induces aresistive component into the impedance of winding lfib and develops anin-phase unbalance signal from bridge 20. Phase-shifter 35 is thenadjusted for maximum D.-C. signal from detector 33, and phase-shifter 26is adjusted for minimum D.-C. signal from detector 24. To provide acheck and to'assure a more-accurate adjustment, a capacitor issubstituted for the resistor in series with the two or three-turn loopof copper wire. This capacitor should have a reactance at the lowerfrequency about equal in magnitude to the ohmic value of the resistor itreplaces. Such a load induces a reactive component in the impedance ofwinding lfib and develops an out-of-phase unbalance signal from bridge20. Phase-shifter 35 is then adjusted for minimum D.-C. signal fromdetector 33, and phase-shifter 20 is adjusted for maximum D.-C. signalfrom detector 20. Little or no readjustment will be required with thisreactive unbalance after adjustment with the resistive unbalance. The.two or 'threeturn loop is removed and the system is now ready for usein the manner already described for the measurement with high precisionof the conductivity and the susceptibility of the earth formations.

The cable l'l preferably includes a strength member for suspension andmovement of the exploration unit in the bore hole l l. The apparatusdiagrammatically shown in Fig. 3 below the fragmentary part of cable d"!is compactly assembled to move as a unit with the coil assembly [0, onlysix conductors being necessary to the illustrated embodiment of theinvention.

The frequencies of 1,000 cycles and 10,000 cycles per second are to betaken as suggestive only and not as limiting the invention since manydifferent frequencies may be utilized. However, in accordance withtheinvention, wide separation of the frequencies utilized isadvantageous. The single magnitude separation,as L000 cycles and 10,000cycles, reduces cross-feed as between the detectors 2% and 33 by twomagnitudes. This is important since the in-phase and quadraturezphasecomponents at the respective frequencies will both change as theconductivity and susceptibility of the earth formations change. However,there will be a selective and disproportionate change in outputs ofdetectors 24 and 33. For example, it will be assumed that the ratio ofthe conductivity of the lower earth formation 12 to the conductivity ofthe upper earth formation I3 is equal to the ratio of the susceptibilityof the upper formation it to they susceptibility of the lower earthformation l2.

The winding ldb in passing from a position adjacent the lower earthformation l2 to a position adjacent the upper earth formation 13 will,of course, produce achange in the output .of-t'he bridge 20. If in termsof Equationb only the foregoing changes be considered, the -quad raturecomponent of the-unbalance at the output of detector 24 will be 10 timesgreater than the quadrature component at detector 33, provided the samegains are used in both channels. Similarly, the inp-hase component ofdetector 33 will be times greater than the inphase component of detector20.

The foregoing will be plainly seen by again referring to Equation 6where the conductivity effects,

term, are proportional to the square of frequency, and the susceptibiliteffects are proportional to the first power of frequency (1X11 term ofthe equation).

The degree to which the bridge 20 may be balanced and the ease ofadjustment to best balance are dependent upon the approach of thecomponents to pure inductance, capacitance and resistive elements. Whilesatisfactory operation may be achieved with the arrangements of Figs. 1and 3, optimum operation may be realized with the embodiment of theinvention shown in Figs. 4 and 5.

Referring now to Fig. 4, an inductance coil assembly 2| is shownpositioned in bore hole ll adjacent an earth formation l2 located justbelow an earth formation l3 having different electrical characteristics.Coil 21 is a two-winding unit, having winding Zia for the lowerfrequency and winding Zlb for the higher frequency. Conductors 5! and 52extend upwardly from winding 2m, and conductors 53 and 5d extendupwardly from winding 2lb. Winding Zia is of comparatively short lengthand large diameter, and is positioned at about the center, lengthwise,of the longer winding 2H9, which is a smaller diameter winding of alength not exceeding about eight times the diameter of the bore hole.Winding 2 la is supported and insulated from winding 2 lb by material2lc, while winding 2lb is supported and insulated on coil form 2 lg.Core 2| f is of a material of high magnetic permeability selected frommaterials suitable for magnetic cores at the frequencies supplied tocoil 2!. Powdered iron, ferrite, and iron strips finely laminated alonglines parallel to the axis of coil 2| are typical suitable materials forthe core 21/. As stated for inductance coil assembly Hi, this core ofhigh permeability material both increases the magnetic flux in thematerial surrounding coil 2| and concentrates most of the reluctancechanges in the magnetic paths in the material surrounding coil 2|. Thisincrease in flux and concentration of reluctance change in thesurrounding material increases the sensitivity of inductance coilassembly 2! to external changes.

Winding 2 la is proportioned to provide a maximum ratio of inductance toresistance due to winding dimensions. In other words, the Q of windingZla is made large, preferably near its maximum value. Winding am, beingof larger diameter and shorter in length than coil 2 lb, will have lessleakage flux and therefore is more susceptible to magnetic contrasts inthe surrounding medium. Although winding 2 la is much shorter thanwinding 2 lb, the number of turns in winding 2 la is much greater thanin winding 2 lb. This is done to make the bridge arm impedanceapproximately the same bridge-20 and bridge 20a. The bridge armimpedance, generally in the order of 1000-5000 ohms, isa-comprornise-between, using high impedance for small valuesofzcondenser 20c v of L22.

and 20g against using high impedances and being subject to straycapacitance unbalancing the bridge.

Winding 2Ib is relatively long compared with the diameter of the borehole in order that the current sheath linked thereto in the surroundingmedium or earth formation shall be as large as possible. This currentsheath is represented in Fig. 4 as an equivalent single-turn secondarywinding 2le. The resistive loading caused by th conductivity of thesurrounding medium is represented by equivalent series resistance 2 Id.

While equivalent resistor 2 Id will vary markedly with changes in theelectrical conductivity of the earth formations surrounding coil 2|, thefull effect of this change does not appear across conductors 53 and 54because it must be transferred thereto through the inductive coupling ofthe equivalent transformer having a secondary winding 2Ie and primaryWinding 2Ib. Because the coefficient of coupling between these windingsis less than 100%, a considerable portion of the change in equivalentresistance Zld will be lost to the measuring circuit connected toconductors 53 and 54. From examination of the second term co M Rgg 2+(Lz2) of Equation 6, it will be seen that this induced resistivecomponent varies with the square of the frequency applied. Accordingly,the higher the frequency applied to conductors 53 and 54 for measurementof this change in electrical conductivity, the more effective thetransfer of resistance changes from the equivalent secondary circuit toprimary winding 2Ib becomes. Accordingly, the higher frequency isapplied to the Winding 2 lb.

Equation 6 expresses the fundamental impedance relationships for bothinductance coil l and inductance coil 2|. However, in the case of thetwo-winding inductance coil 2|, a separate Equation 6 would be requiredto express the impedance relationships for each winding 2Ia and 2 II).For winding 2 la, the first term, iXn, is the important term inasmuch asit expresses the primary reactance which will change with variations inprimary inductance as the magnetic susceptibility of the surroundingearth formation changes. The signal due to the second term,

(.0 M 2R R32 22 2 is excluded by the phase-sensitive action of detector24, while the third term,

jw M 1122 z'i f'2) can be neglected because of the very low value Havingshown that the same basic theory underlies the two-winding embodiment ofthe inductance coil assembly, the embodiment illustrated in Fig. 5 willbe described in detail. Inductance coil assembly 2| is schematicallydisplayed, with winding 2Ia shown as more influenced by the variationsin the magnetic susceptibility of the path surrounding winding Zla thanby variations in the electrical conductivity in the equivalent secondarycircuit consisting of equivalent winding 2 le and equivalent resistance2Id. Winding 2lb is schematically displayed as more influenced by thetransformer relation to equivalent secondary winding 2 le and equivalentsecondary resistance 2Id than by variations in the magneticsusceptibility of surrounding material. In order to accentuate theresponse due to changes in the electrical conductivity of thesurrounding earth formations, winding 2Ib is energized with alternatingcurrent of high frequency, such as 10,000 cycles per second. Anysuitable source 49 of such high frequency may be utilized. It issupplied to winding 2Ib by way of a bridge 20a.

In order to obtain the response to changes in magnetic susceptibility ofthe surrounding earth formations, winding 2Ia is energized withalternating current of low frequency such as 1,000

' cycles per second, Any suitable source 48 of such low frequency may beutilized. It energizes winding 2Ia by way of bridge 20. By utilizingseparate frequencies for the measurement of changes due to magneticsusceptibility and of changes due to changes in electrical conductivity,greater resolution of the two changes is obtained with less lized withthe particular winding of inductance coil 2I that provides the signal ofa particular phase and frequency, indicative of the variation to bemeasured.

When bridge 20 is unbalanced by changes in the impedance of winding 2Ia,a low-frequency unbalance signal is developed by bridge 20. Filter 22applies this low-frequency signal to amplifier 23 and excludes thehigh-frequency voltage. Th output of amplifier 23 is applied tophase-sensitive, frequency-selective detector 24 as described inconnection with Fig. 3. The voltage supplied from low-frequency source48 through phase-shifter 26 to detector 24 is adjusted in phase byphase-shifter 26 to be inphase with the quadrature or out-of-phasecomponent of the voltage applied to detector 24 by amplifier 25.

The quadrature component of the low-frequency voltage output from bridge20 varies with the susceptibility of the earth formations surroundinginductance coil 2 I, so the susceptibility upon the impedance of winding2 Id.

The output from detector 24 may be connected directly to an indicatingand/or recording instrument or, as shown in Fig. 3, by way of anamplifier 45 to a recorder 30.

When bridge 20a is unbalanced by changes in the impedance of winding21b, an in-phase, highfrequency unbalance signal will be developed bybridge 20a proportional to the impedance changes a-cagcss in winding2|b-and the changes 'inrelectrical conductivity of theearthformationssurrounding inductance coil 2|. Filter 3| applies thehigh-frequency output from bridge 20a to amplifier 32 and excludes thelow-frequency voltages. The output of amplifier 32 is applied todetector 33 similar in design and operation to the-detector 24 andhaving the circuits shown for detector 33 in Fig. 3. Phase-shifter 35applies some of the voltage from high-frequency source- 49 to detector33, and phase-shifter 35 is adjusted so that the voltage applied todetector 33 is in phase with the in-phase component of the highfrequencyvoltage from bridge 20a. Since this in-phase, high-frequency voltage isrepresentative of changes in the electrical conductivity of the earthformations surrounding inductance coil 2| and the resulting impedancechange in winding 2|b, there is produced from detector 33 adirect-current output whose magnitude will vary with the inducedresistive component of winding 2|b which, in turn, will depend upon theconductivity of the earth formations with which it is inductivelycoupled.

While the outputof detector 33 may be directly connected to indicatingand/or recording apparatus, it can be connected to an amplifier 46 foroperation of the marker producin trace 300 in recorder 33' as shown inFig. 3. As described for Fig. 3', trace 38s produced by recorder 39 inresponse to direct-current output from detector 24 and trace 33cproduced in response to output from detector 33, respectively willindicate the changes in magnetic susceptibility and electricalconductivity of 'theearth formations surround-- ing inductance coil 2|as a function of its position in the earth.

In the foregoing description of the operation of Fig. 5, it was tacitlyassumed that the balanceable circuits 2B and 22a had been adjusted forproper operation. The balancing of bridge 20 is relatively easy toaccomplish by proper adjustment of the values of variable resistor 20dand of variable capacitor 23c. Bridge 23a is similarly balanced byadjustment of the values of variable resistor 23 and of variablcapacitor 20g.

Since windings 2|a. and 2|b are commonly coupled to magnetic core 2| 1,it will be obvious to those skilled in the art that the loadingwhichwinding Zia sees between conductors 3| and 32, that is, the impedance offilter 22 in series with the resistor 23h of bridge 20, will bereflected by transformer action into winding 211). Similarly, theloading which winding-2H2 sees between conductors. 33 and 34; that is,the impedance of filter 3| in series with resistor 231' of bridge-20a,will be reflected by transformer action into winding 2| :1. Accordingly,it'becomes highly'desirable to have both filter 22 and filter 3|reject-j those frequencies which are objectionable.

The arrangement of Fig. 3, wherein oscillators Q8 and 49providethelovv-frequency and highfrequency voltages'from the surface to theexploration unit in the bore hole via conductors 42 and 33, can beutilized in Fig. 5 if desired. However, thealternate arrangement shownis utilized, wherein the sources of low-frequency and high-frequencyvoltages are contained withinthe bomb or exploration unit and conductors42 and 43 serve only to supply power voltage to these oscillators.

With inductance coil 2! suspended in air, a resistive component isinduced into the impedance: of windings 2|a and 2|b by placing a two orthree-turn copper' loop around inductance coil 2| with resistance inseries therewith whose ohmic value is high compared to the reactanceof'the loop at the frequencies used. Theimpedance of this secondary willbe predominantly resistive, and hence the impedance change induced intothe windings of inductance coil 2| also will be predominantly resistive.As this impedance change unbalances networks 20 and 26a, an outputsignal will appear. The high-frequency portion of the signal from bridge23a will be passed by filter 3|, amplified by amplifier 32 and appliedto phase-sensitive detector 33. Since this is the in-phase signal forwhich detector 33 should produce a D.-C. output, phase-shifter 35 isadjusted until the output of detector 33 is at a maximum.

The low-frequency output from network 20 will be passed by filter 22,amplified by amplifier 23' and applied to phase-sensitive detector 24.Since this low-frequency signal is the in-phase component, it is notdesired that detector 24 present a D.-C. output for this phase ofsignal. Accordingly, phase-shifter 23 is adjusted for'minimum.D.-C.output. from detector'24.

A quadrature signal is developed by substituting a capacitor for theresistor in series with the two or three-turn loop. The capacitor shouldhave a reactanceabout equal to the ohmic value of the resistor itreplaces. This secondary will induce a reactive component in thewindings of inductance coil 2|, and will cause an unbalance signal toappear on the outputs of balanceable networksill and 20a. This outputwill be the quadrature-signalwhich in actual measurement represents achange in magnetic susceptibility. It will be found that thehigh-frequency signal applied to detector 33 will be substantially outof phase with the signal applied through phaseshifter 35, so that littleor no adjustment of phase-shifter 35 will be required to provide minimumD.-C. output from detector 33 for this quadrature component signal. Thelow-frequency signal applied to detector 24 will be found to besubstantially in phase with the signal applied through phase-shifter 26,so that little or no readjustment of phase-shifter 26 will be requiredto provide maximum output from detector 24.

Phase-sensitive detectors 24 and 33 are well known in the art. Theiroperation is described on pages 206-209 of Servomechanism Fundamentalsby Lauer, Lesnickand Matson (first edition published 1947 by McGraw-HillBook Co. Inc.). Briefly, detector 24 includes rectifiers 24a and 24b,and output resistor 240. The rectifiers 24a and Z ib are similarly poledwith respect to thereferencevoltage applied to transformer 24d andareoppositely poled with respect to the signal voltage acrossthe outputfrom phase-shifter 26. The operation of the rectifiers is such that theD.-C. output across resistor 240 is proportional to the magnitude of thesignal applied to transformer-23d which is in phase with the referencevoltage from phase-shifter 26.

The circuit of detector'33 is the same both in constructionand operationas detector 24. The D.-.C. output across resistor-33c is proportional tothe magnitude of the signal applied to transformer 33d which is in phasewith the reference voltage from phase-shifter 35.

When the initial adjustments of phase-shifters 26 and 35 have beencompleted, with inductance coil 2| suspended in air, the bore-holeexploration unit is ready for lowering into a bore hole for loggingpurposes. When lowered into a bore hole, inductance coil 2! ismagnetically coupled to the surrounding medium. The changes in itsimpedance produced by resistive and reactive components being inducedtherein by this coupling to the surrounding medium will be separated,detected and recorded as a function of the position of the explorationunit in the bore hole.

What is claimed is:

l. A system for measuring magnetic susceptibility and electricalconductivity comprising supply means for alternating voltage ofdifferent frequencies, balanceable circuit means connected to saidsupply means, inductive means connected to said balanceable circuitmeans and magnetically coupled to a surrounding medium, phasesensitivedetecting means responsive to an alternating voltage of one frequencyconnected to said circuit means and to said supply means to receive analternating voltage of one frequency, and phase-sensitive detectingmeans responsive to alternating voltage of a different frequencyconnected to said circuit means and to said supply means to receive analternating voltage of said different frequency, said detecting meansmeasuring the unbalance signal of said circuit means in terms of theout-of-phase component of one frequency and the in-phase component ofthe different frequency whereby the magnetic susceptibility andelectrical conductivity of the medium surrounding said inductance coilis measured.

2. A system for measuring the magnetic susceptibility and electricalconductivity of earth formations in a bore hole comprising inductivecoupling means adapted for movement adjacent said earth formations inmagnetically coupled relation therewith, voltage supply means forapplying to said inductive coupling means alternating voltages at twofrequencies of widely differing magnitudes, an electrical circuitincluding a filter for passing alternating current of a first of saidfrequencies and for excluding alternating current of the second of saidfrequencies, another electrical circuit including a filter for passingalternating current of said second frequency and excluding alternatingcurrent of said first frequency, means including at least onebalanceable network connected between said filtering means and saidinductive coupling means and to aid voltage supply means, and means incircuit with the output from each of said filters for respectivelyindicating the in-phase component of the alternating current of onefrequency and for indicating the out-of-phase alternating currentcomponent of the other frequency in measurement respectively of themagnetic susceptibility and the electrical conductivity of the earthformations adjacent which said inductive coupling means may be disposed.

3. A system for measuring magnetic susceptibility and electricalconductivity of earth formations in a bore hole comprising inductivecoupling means adapted for movement adjacent said earth formations inmagnetically coupled relation therewith, alternating-current supplymeans for applying to said inductive coupling means alternating currentsof substantially different frequencies, circuit means including at leastone balanceable network responsive to variations in the impedance ofsaid inductive coupling means caused by changes in the electrical andmagnetic characteristics of said earth formations coupled thereto,electrical circuits connected to said circuit means, one including afilter for passing alternating current of said one frequency and forexcluding alternating current of said other frequency, anotherelectrical circuit including a filter for including alternating currentof said second frequency and excluding alternating current of said firstfrequency, and means in circuit with the output from each of saidfilters for respectively indicating the iii-phase component of the al--ternating current of one frequency and for indicating the out-of-phasealternating current component of the other frequency in measurementrespectively of the magnetic susceptibility and of the electricalconductivity of the earth formations adjacent which said inductivecoupling means may be disposed.

4. A system for measuring magnetic susceptibility and electricalconductivity comprising a source of alternating voltages at differingfrequencies, balanceable circuit means connected to said source ofalternating voltages, inductive coupling means connected to saidbalanceable circuit means and magnetically coupled to a surroundingmedium, phase-sensitive detecting means responsive to a first frequencyoutput from said balanceable circuit means having a particular phaserelation to the first frequency output from said source of alternatingvoltages, phase-sensitive detecting means responsive to a secondfrequency output from said balanceable circuit means having a particularphase relation to the second frequency output from said source ofalternating voltages, whereby the output from said phase-sensitivedetecting means respectively are indicative of the magneticsusceptibility and of the electrical conductivity of the mediumsurrounding said inductance coupling means.

5. Means for measuring the conductivity and susceptibility of earthformations comprising inductive means supported in magnetically coupledrelation to an earth formation, means including a balanceable electricalcircuit for applying to said inductive means alternating currents ofwidely differing frequency, said circuit being unbalanced with changesin conductivity and in susceptibility of said formation, means includinga phase-sensitive detector for measuring the inphase component of theunbalance signal of the higher frequency alternating current indicativeof said change in conductivity, and means including a phase-sensitivedetector for measuring the quadrature-phase component of the unbalancesignal of the lower frequency alternating current as indicative of saidchange in susceptibility.

6. Means for measuring the conductivity and susceptibility of earthformations comprising inductive means supported in magnetically coupledrelation to an earth formation, means including a balanceable electricalcircuit for applying to said inductive means alternating currents ofwidely differing frequency, said circuit being unbalanced with changesin conductivity and in susceptibility of said formation, means includinga phase-sensitive detector for measuring the inphase component of theunbalance signal of the higher frequency alternating current indicativeof said change in conductivity, and means including a phase-sensitivedetector for measuring the quadrature-phase component of the unbalancesignal of the lower frequency alternating current as indicative of saidchange in susceptibility, the in-phase component of said lower frequencyalternating current and the quadrature-phase component of saidhigh-frequency alternating current being of a materially lower order ofmagnitude than their respective components representative of magneticsusceptibility and of electrical conductivity.

'7. In a system for measuring the conductivity and susceptibility ofearth formations, inductive means positioned in magnetically coupledrelation to an earth formation, means including balanceable circuitmeans for applying to said inductive means alternating currents ofwidely differing frequency, said circuit means being unbalanced withchange in conductivity and in susceptibility of said formation, firstfrequency-selective means including a phase-sensitive detectorfor'measuring the in-phase component at a first frequency of theunbalance signal from said circuit means indicative of said change inconductivity, and second frequency-selective means including aphase-sensitive detector for measuring the quadrature-phase component ata second frequency of the unbalance signal from said circuit meansindicative of said change in susceptibility, the wide frequencyseparation of the unbalance signal at the first frequency and theunbalance signal at the second frequency providing considerableseparation of the susceptibility and conductivity effects in theirrespective phasesensitive detectors.

8; An apparatus for the measurement of changes in magneticsusceptibilityand in electrical conductivity of earth formations, comprising a sourceof low-frequency current, a source of' high-frequency current, inductivecoupling meansenergized by said sources and magnetically coupled to theearth formation to be measured, and measuring means responsive toreactive changes in the impedance of said inductive coupling means tolow-frequency current and responsive to resistive changes in theimpedance of said inductive coupling means to high-frequency current,said reactive changes being representative of changes in magneticsusceptibility of earth formations and said resistive changes beingrepresentative of changes in electrical conductivity of earthformations.

9. An apparatus for the measurement of changes in electricalconductivity and magnetic susceptibility of a material, comprising asource of high-frequency voltage, a source of low-frequency voltage,inductive coupling means electrically excited by said sources ofhigh-frequency and of low-frequency voltage and magnetically coupled tothe material to be measured, measuring means responsive to reactivechanges in the impedance of said inductive coupling means at lowfrequency, said reactive changes being representative of changes in themagnetic susceptibility of the material, and measurin means responsiveto resistive changes in the impedance of saidinductive coupling means athigh frequency, saidresistive changes being representative of changes inthe electrical conductivity of the material.

10. A system for measuring changes in magnetic susceptibility andelectrical conductivity comprising a source of a plurality ofalternating voltages, balanceable circuit means connected to said sourceof alternating voltages, an inductance coil connected to saidbalanceable network and responsive to said alternating-voltages, saidinductance coil being coupled to the material to be measured wherebyreactive changes are induced in the impedance of the inductance coil andare representative of the changes in magnetic susceptibility and wherebyresistive changes are induced in the impedance of said inductance coiland are representative of changes in electrical conductivity,phase-sensitive detecting means responsive to a low-frequency outputfrom said balanceable network representative of changes in magneticsusceptibility, and phase-sensitive detecting means responsive to ahigh-frequency output from said balanceable network representative ofchanges in electrical conductivity, whereby voltages representative ofchanges in magnetic susceptibility and in eiectrical conductivity areprovided for indication and recording.

11. A system for logging bore holes by measuring the changes with depthof magnetic susceptibility and of electrical conductivity of the mediumsurrounding the core hole and traversed by the bore hole, comprising abomb adapted to be lowered into said bore hole, a balanceable networkarranged in said bomb and adapted to receive a plurality of alternatingvoltages from sources thereof on the surface, an inductance coilarranged in said bomb, said inductance coil being connected to saidbalanceable network and magnetically coupled to the surrounding mediumwhereby reactive and resistive components are induced into the impedanceof said inductance coil, and a plurality of phase-sensitivefrequencyselective detectors arranged in said bomb and responsive toparticular alternatin voltages in the output from said balanceablenetwork, said particular voltages being respectively representative ofthe changes in magnetic susceptibility and in electrical conductivity ofthe surrounding medium, conductors extending from said detectors to thesurface, and means at the surface for indicating the changes in magneticsusceptibility and in electrical conductivity as a function of theposition of the bomb in the bore hole. 12. In an electromagnetic borehole logging system, a system for resolving signals representative ofmagnetic susceptibility and electrical conductivity, comprisin aninductance coil having a high-frequency winding and a low-frequencywinding, a source of high-frequency voltage connected to saidhigh-frequency winding, a source of low-frequency voltage connected tosaid low-frequency winding, balanceable circuit means connected to saidsources of high-frequency and low-frequency voltages and to saidinductance coil, phase-sensitive detecting means connected to the outputof said balanceable network and to said source of low-frequency voltagesfor measurement of low-frequency output from said balanceable networkhaving a particular phase relation to said low-frequency voltage source,phase-sensitive detecting means connected to the output of saidbalanceable network and to said source of high-frequency voltage formeasurement of a high-frequency output from said balanceable networkhaving a particular relation to said source of high-frequency voltage,whereby variations in the magnetic susceptibility and electricalconductivity of the material surrounding said inductance coil aremeasured.

13. A system for measuring magnetic susceptibility and electricalconductivity of a medium surrounding a bore hole comprising a source oflow-frequency voltage, a source of high-frequency voltage, a firstbalanceable network responsive to the low-frequency voltage andconnected to said source of low-frequency voltage, a second balanceablenetwork responsive to the highfrequency voltage and connected to saidsource of high-frequency voltage, an inductance coil having a firstwinding connected in the first balanceable network and a second windingconnected in the second balanceable network, said inductance coil beingmagnetically coupled to the medium being measured to induce reactivechanges in the first Winding representative of changes in magneticsusceptibility and to induce resistive changes in the second windingrepresentative of changes in electrical conductivity, afrequency-selective, phase-sensitive detector responsive to alow-frequency output from the first balanceable network, which output isindicative of changes in magnetic susceptibility, and afrequency-selective, phase-sensitive detector responsive to ahigh-frequency output from the second balanceable network, which outputis indicative of changes in electrical conductivity.

14. An apparatus for the measurement of changes in electricalconductivity and in magnetic susceptibility of a material comprising asource of high-frequency voltage, a source oi low-frequency voltage,inductive coupling means magnetically coupled to the material to bemeasured and having a first coil excited by said source ofhigh-frequency and a second coil excited by said source oflow-frequency, measuring means responsive to resistive changes in theimpedance of said first coil at high frequency, said resistive changesbeing representative of changes in electrical conductivity of thematerial, and measuring means responsive to reactive changes in theimpedance of said second coil at low frequency, said reactive changesbeing representative of changes in magnetic susceptibility of thematerial.

15. An apparatus for the measurement of changes in lectricalconductivity and in magnetic susceptibility of a material comprising asource of high-frequency voltage, a source of lowfrequency voltage,inductive coupling means magnetically coupled to the material to bemeasured and having a first coil whose length is several times longerthan its diameter and which is excited by said source of high-frequencyand having a second coil whos length is comparable to the diameter ofsaid first coil and which is excited by said source of low-frequency,said second coil being symmetrical about the axis of said first coil,measuring means responsive to resistive changes in the impedance of saidfirst coil at high-frequency, said resistive changes beingrepresentative of changes in electrical conductivity of the material,and measuring means responsive to reactive changes in the impedance ofsaid second coil at low frequency, said reactive changes beingrepresentative of changes in magnetic susceptibility of th material.

ROBERT A. BRODING.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,938,534 Peters Dec. 5, 19332,018,080 Martienssen Oct. 22, 1935 2,033,046 Jakosky Apr. 21, 19362,535,666 Broding Dec. 26, 1950

